I like the button in general, however as I am left handed it happens regularly that I accidentally hit the back to top button while scrolling. I suggest adding an option to move the floating button to the other side of the screen.
The floating button is currently being overhauled which I think should make changing the position easier even if it doesn’t support it out of the gate.
In the mean time, you could turn off the floating button for the feed view and instead use the feed button on the bottom navigation bar to scroll back to top. Bit of a hidden functionality :) Don’t think there is currently a work around for the post view though unfortunately.
It might be a good idea to hide the button until a user scrolls up a little first. This way the users intent is clear, they either want to scroll up a little or a lot.
Great idea!